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Regulatory Toxicology and Pharmacology xxx (2008) xxx–xxx
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Risk assessment due to environmental exposures to fibrous
particulates associated with taconite ore
Richard Wilson a,*, Ernest E. McConnell b, M. Ross c, Charles W. Axten c, Robert P. Nolan c
a
Department of Physics and the Center for Risk Assessment, 9 Oxford Street Rear, Harvard University, Cambridge, MA 02138, USA
b
ToxPath, Inc., 3028 Ethan Lane, Laurdale Estates, Raleigh, NC 27613, USA
c
Center for Applied Studies of the Environment and Earth and Environmental Sciences, The Graduate School and University Center,
The City University of New York, 365 Fifth Avenue, New York, NY 10016, USA
Received 25 October 2007
Abstract
In the early 1970s, it became a concern that exposure to the mineral fibers associated taconite ore processed in Silver Bay, Minnesota
would cause asbestos-related disease including gastrointestinal cancer. At that time data gaps existed which have now been significantly
reduced by further research. To further our understanding of the types of airborne fibers in Silver Bay we undertook a geological survey
of their source the Peter Mitchell Pit, and found that there are no primary asbestos minerals at a detectable level. However we identified
two non-asbestos types of fibrous minerals in very limited geological locales. Air sampling useful for risk assessment was done to determine the type, concentrations and size distribution of the population of airborne fibers around Silver Bay. Approximately 80% of the
airborne fibers have elemental compositions consistent with cummingtonite-grunerite and the remaining 20% have elemental compositions in the tremolite-actinolite series. The mean airborne concentration of both fiber types is less than 0.00014 fibers per milliliter that is
within the background level reported by the World Health Organization. We calculate the risk of asbestos-related mesothelioma and
lung cancer using a variety of different pessimistic assumptions. (i) that all the non-asbestos fibers are as potent as asbestos fibers used
in the EPA-IRIS listing for asbestos; with a calculated risk of asbestos-related cancer for environmental exposure at Silver Bay of 1 excess
cancer in 28,500 lifetimes (or 35 excess cancers per 1,000,000 lifetimes) and secondly that taconite associated fibers are as potent as chrysotile the least potent form of asbestos. The calculated risk is less than 0.77 excess cancer case in 1,000,000 lifetimes. Finally, we briefly
review the epidemiology studies of grunerite asbestos (amosite) focusing on the exposure conditions associated with increased risk of
human mesothelioma.
! 2008 Published by Elsevier Inc.
Keywords: Risk; Fibers; Particles; Taconite; Asbestos; Exposures
1. Introduction
A risk assessment for the effects of fibrous particles in
taconite ore is simultaneously very simple and somewhat
complex. It is simple because there have been good epidemiological studies of the health of miners, workers, and
nearby residents, that have been exposed to such particulates at historically higher concentrations than exist today
*
Corresponding author. Fax: +1 617 332 4823.
E-mail address: wilson5@fas.harvard.edu (R. Wilson).
(Ross et al., 1993; Brunner et al., 2008; Gamble and Gibbs,
2008). No statistically significant increase in cancer risk has
been found due to fibrous particulates commonly associated with taconite ore. Although sufficient time has passed
to allow for the long latency commonly associated with
human cancer. In the intervening years dust concentrations
both in air and water have been much reduced—perhaps by
a factor of 50. Since adverse effects are expected to be
reduced at least as much as the concentrations although
zero divided by 50 is still zero, one can estimate thereby
that there will be no directly measurable risk.
0273-2300/$ - see front matter ! 2008 Published by Elsevier Inc.
doi:10.1016/j.yrtph.2007.11.005
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But this simple argument, while correct, is inadequate to
satisfy legitimate concerns of public health authorities and
of public emotions. This we see as follows. An epidemiological study of a new agent has never been accepted as sole
evidence that this agent causes adverse health effects unless
the probability of an adverse outcome is at least doubled.
Technically this means that the Risk Ratio, or RR, must
be greater than 2, if an agent has already been accepted
as the cause of adverse effects at higher concentrations,
then a RR as low as 1.3 has sometimes been accepted as
evidence of increased risk. For example, RR > 1.3 has been
accepted as evidence that second-hand cigarette smoke
causes cancer since it is well known that cigarette smoking
causes cancer. EPA has accepted a much smaller RR of
1.05 as evidence for an effect of air pollution since we
know, for example, that in December 1952, over 4000 people died in London with RR > 2. A RR > 1.3 is accepted
that X-rays in pregnancy can cause childhood leukemia
since radiation is known to be dangerous.
But few would accept a Risk Ratio of 1.05 as evidence
by itself of adverse effects. Below we estimate for residents
of Silver Bay a much smaller RR of 1.0005, (risk of
4 ! 10"5) which obviously cannot be measured by direct
epidemiological evidence. Yet in 1975, when risks from
the taconite mines was first being seriously discussed, the
US EPA was trying to regulate risks at a one in a million
per lifetime level, pessimistically calculated. For lung cancer that was a Risk Ratio of 1.000013.
2. Origin of the problem
Concern about exposure to fibrous minerals at Northshore Mining Company originated when Reserve Mining
Company began an effort to commercially process the taconite iron ore from the Peter Mitchell Pit. Northshore’s
operation is located in the eastern part of Minnesota’s
Mesabi Iron Range which contains vast quantities of taconite ore. Processing the ore to pellets suitable for commercial sale required more water for the wet magnetic
separation than was available at the mine site and so
Reserve Mining decided to move the iron ore by rail to
the shore of Lake Superior and develop a processing facility at that site, i.e., Silver Bay, Minnesota. In 1948 the
Army Corp of Engineers issued the required permit to
Reserve Mining for disposal of the waste rock generated
from the processing facility in a deep trough (900 feet) in
Lake Superior off-shore from Silver Bay (Bartlett, 1980).
The Northshore Mining Company began operating the Silver Bay facility and Peter Mitchell Pit in 1994.
Lake Superior supplied all of the water the facility
required and the iron ore product could then be transported by ship. The initial plan called for disposing of
the waste rock into Lake Superior, which would eventually
reach 67,000 tons/day. Concern initially focused on the
possibility that the waste rock would have long-term
adverse effects on the ecology of the lake (Bastow, 1986).
Also of concern was the light scattering from the fine par-
ticles suspended in the water which caused surface clouding
and discoloration sometimes appearing as green water
caused by the movement of the particle plume. Analysis
by X-ray diffraction and transmission electron microscopy
of these suspended particulates revealed the presence of
particles suspended in the lake water a percentage of which
were reported to be asbestiform amphiboles (Cook et al.,
1974). The predominant fibrous amphibole in the water
was reported to be in the same cummingtonite-grunerite
series as we found in the air samples collected about 25
years later. However, we did not find the population of
fibers to have morphological characteristics consistent with
grunerite asbestos or any other type of amphibole asbestos.
Cook et al., 1974 reported that analysis by X-ray diffraction revealed about 23% of the suspended particles in the
lake were amphiboles but the type of amphibole and the
percentage that were asbestiform are not reported. These
early studies did not include a geological survey of the mine
to identify the origin of the asbestiform fibers. Generally
air and water samples from other locations were used as
negative controls to determine if the levels fibers in Silver
Bay were increased.
The taconite ore contains approximately 30% amphibole
minerals which are a group of silicates commonly found in
the earth’s crust. In addition to quartz, three types of
amphiboles were identified as predominantly present—
hornblende, cummingtonite-grunerite and tremolite-actinolite. Amosite is the commercial name given to grunerite
asbestos. Analysis of airborne fibrous particles by analytical transmission electron microscopy indicated elemental
compositions indistinguishable from two amphibole minerals that can occur as asbestos, cummingtonite-grunerite
and tremolite-actinolite were present meriting further consideration. At the time attention was focused on the cummingtonite-grunerite fibers and if exposures to them
particularly in potable water present a risk similar to amosite asbestos where exposure to airborne dust for just one
year increased the rate of gastrointestinal cancer (Selikoff
et al., 1972). Whether or not the health hazards of these
two types of amphiboles were the same as those of asbestos
fibers was not clearly established. Of less concern was the
presence of tremolite-actinolite fibers at that time little evidence of health effects existed and of least concern was
hornblende which can form fibers but rarely, if ever, forms
asbestos and other minerals commonly found in ambient
air (Langer et al., 1979; Veblen and Wylie, 1993).
During this same time period in the early 1970s, workers
with heavy occupational exposure to commercial asbestos,
were found by epidemiological studies to have more gastrointestinal cancer than would normally be expected (Selikoff
et al., 1972; Selikoff and Hammond, 1979). These important findings stimulated questions about the potential for
the fibrous amphiboles discarded into Lake Superior to
increase the risk of gastrointestinal cancer. As several communities used unfiltered water from Lake Superior for
drinking, there was concern that ingesting fibrous particles
associated with taconite could increase the risk of develop-
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R. Wilson et al. / Regulatory Toxicology and Pharmacology xxx (2008) xxx–xxx
ing gastrointestinal cancer in the same way that the asbestos workers had increased risk from exposure to similar
mineral fibers by inhalation (Hills, 1979). Studies indicate
that the potential ingestion of these fibers did not pose
the same level of risk that was affecting asbestos workers
(Moore, 1978).
The grunerite asbestos workers were exposed to markedly higher concentrations airborne fiber than the workforce at Reserve Mining (Nolan et al., 1999; Ribak and
Ribak, 2008). In addition the gastrointestinal tract of
asbestos insulation workers had been exposed by inhalation either by directly swallowing airborne fibers or by
coughing up and swallowing inhaled asbestos fibers leading
to an increased risk of gastrointestinal cancer. The exposure from Lake Superior would be primarily from drinking
water containing mineral fibers although concern was also
expressed about the potential of inhalation exposures from
the re-entrainment of fibers remaining on the floor after the
water from Lake Superior used to wash them evaporated.
Inhalation is a more common route of asbestos exposure
and the health effects are better understood and might produce asbestos-related disease in the general population
(Mason et al., 1974; Levy et al., 1976; Sigurdson et al.,
1981; Sigurdson, 1983).
The findings of Selikoff (1974) properly raised the public
health concern about asbestos. Workers with occupational
asbestos exposures could be experiencing up to 25% excess
mortality from asbestos-related diseases (Selikoff et al.,
1979). Two points learned in these studies underscored the
concern about the exposures in Silver Bay. Firstly, the asbestos-related cancers have a very long latency period with little,
if any, disease occurring less than 20 years after first being
exposed. Therefore any disease which might be associated
with disposing of the waste rock in the lake would not be
observable for a considerable period of time. The second
concern was the build-up of fibers in the lake water that could
turn out to be a human carcinogen. If the concentration of
fibers in the lake water increased with time so that in 20
years—when the increased risk of gastrointestinal cancer
would be expected to become observable—fewer options
would be available to reduce the risk of fiber related gastrointestinal cancer in the already exposed population.
The Eighth Circuit Court of Appeals rendered its decision in the Reserve Case on March 14, 1975 (Reserve Mining, 1975). At that time it was decided to monitor the
airborne and waterborne fibers in Silver Bay using an indirect sample preparation technique and analytical transmission electron microscopy (ATEM). In 1975 there was no
standard in the scientific or medical literature worldwide
to evaluate the non-occupational cancer risks which might
be associated with exposure to the various types of fibrous
minerals being found in and around Silver Bay (National
Research Council, 1984). The measurements of the airborne fibers in the non-occupational environment were
unreliable and no risk assessment models existed to predict
the risk of asbestos-related cancer related to any exposure
measured (Peters and Doerfler, 1978).
3
Ultimately the solution for the Silver Bay facility was to
end the practice of discharging the waste rock into the lake
and dispose of the material in a facility on land. Duluth’s
drinking water was filtered by November, 1976 and the disposal of the waste rock in Lake Superior ended by July,
1980. To limit the exposure to the fibers present in the
waste rock by inhalation, they were to be placed under
water in a large in land disposal basin. Clear evidence of
a public health problem did not drive the steps taken at
that time, but rather the decisions were made based on
extrapolation, judgment and a desire to do no harm
(Schaumburg, 1976; Bartlett, 1980; Bastow, 1986). This
would now be called the Precautionary Principle. It can
be satisfied by an open and effective use of evidence based
risk assessment (Richter and Laster, 2004).
Occupational exposures were monitored by phase-contrast light microscopy counting the number of fibers equal
to or greater than 5 lm in length with a length to width
ratio of 3:1 or greater, reporting the number of such fibers
per milliliter of air (Langer et al., 1991). Occupational
exposure to asbestos is controlled using an index of exposure not by measuring the total number of fibers to which
a worker is exposed. In 1971, the asbestos standard was
12 f/mL and the initial goal of the standard was to eliminate the development of asbestosis. By 1994 the standard
had been lowered on four occasions to the current standard
of 0.1 f/mL mainly to reduce the risk of asbestos-related
cancer (Fig. 1).
The decision in the Reserve Mining Case ordered St. Paul
to be used as a control city to assess Silver Bay exposures.
The theory was that if the airborne concentrations of fibrous
mineral (of all lengths) in Silver Bay were below those for airborne asbestos in St. Paul, then the levels would be presumed
to be safe and further steps to reduce the concentration of airborne fibers in Silver Bay would not be necessary. This control city protocol was not benchmarked to the risk of
asbestos-related cancer, the levels of airborne asbestos in
St. Paul were simply assumed to be safe.
The air samples collected and analyzed to comply with
the 1975 court decision, which continue to this day, are prepared using the indirect method and uses analytical transmission electron microscopy as instrument of choice. The
airborne particles were collected on membrane filters, the
filters were then dissolved and the fibers dispersed in water.
An aliquot of the suspension is than filtered onto a new filter at a lower particle loading. As the air monitoring program began to compare the airborne fiber levels in the
two locations—those in Silver Bay (0.0048 f/mL,
N = 155) were consistently #5-fold lower than in St. Paul
(0.023 f/mL, N = 35) (Fig. 1). The airborne fibers in St.
Paul were predominantly chrysotile asbestos while in Silver
Bay fibrous particle associated with taconite predominated.
Eventually the air sampling in St. Paul was discontinued
and the ongoing air monitoring in Silver Bay is now only
compared to the airborne concentrations of fibers already
determined historically in St. Paul and Silver Bay. In
March of 2006 the Minnesota Pollution Control Agency
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Fig. 1. The airborne concentration of taconite associated fibers greater than 5 lm in length is less than 0.00014 f/mL in Silver Bay, Minnesota which is at
the low-end of background for airborne asbestos concentration and orders of magnitude less than the historically high occupationally exposed of the past.
has started to collect air samples in St. Paul again. Indirect
sample preparation has been largely abandoned although
the analytical transmission electron microscopy remains
the instrument of choice for monitoring the non-occupational environment for asbestos.
For comparison, occupational exposure to airborne
asbestos in 1975 was held below 5 f/mL by regulation, as
determined by phase-contrast optical microscopy (PCOM)
using a direct transfer sample preparation method and
counting only fibers greater than or equal to 5 lm (Fig. 1).
Thus, even if the fibers from Silver Bay were asbestos, the levels monitored in Silver Bay are at least 700 times lower than
the current permissible exposure level for asbestos.
Ambient air in all natural settings contains airborne particulates, among them asbestos, and Silver Bay is no exception. Airborne asbestos has been found on small isolated
Pacific Islands without naturally occurring asbestos and
in 10,000-year-old ice samples from Antarctica indicating
airborne asbestos pre-dates its industrial use (Kohyama,
1989). Bowes et al., 1977 report asbestos to be present in
the Greenland ice cap indicating airborne asbestos was
present in both hemispheres prior to industrial use. Of all
the particles in the air only a small percentage are fibers
and generally a sub-population of these are asbestos. Since
the air in Silver Bay contained very few fibers it was necessary to sample large volumes of air to determine if there
was any asbestos present. The air samples, required by
the court decision, were continuously collected over three
days of sampling the air at 16.7 l of air per minute (Axten
and Foster, 2008).
After this long period of air sampling, the filters on which
the particles deposited are too heavily loaded for direct
examination of the small number of the fibers collected.
The particles collected on the filter, the filter was dissolved
using chemicals or low-temperature ashed and re-dispersed
in water. An aliquot was then filtered onto a new membrane
filter at a lower particle density for counting purposes. By
selecting aliquots of various volumes the loading can be
adjusted to an optimum for fiber counting. The use of the second membrane is called indirect sample preparation and can
alter the particle number and size distribution. The transfer
to the second filter to reduce the fiber density changes the
fiber size distribution and counting fibers of all lengths rather
than only those greater than or equal to 5 lm cause the indirect sample preparation method not to use when air sampling
for the purpose of asbestos risk assessment.
3. Changes in the state of knowledge regarding asbestos and
other fibers
It is now more than 30 years since the 1975 court decision ruling the Reserve discharge into Lake Superior posed
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a potential health threat. Additional information about the
health effects of asbestos and other fibers has become available relevant to the decision in the Reserve case. Even
among those with high-occupational exposure to asbestos
there is no consistent increase in the risk of gastrointestinal
cancer (Gamble, 1994; Weiss, 1995; Gamble and Gibbs,
2008) and therefore no risk assessment model exists. Drinking potable water carried in asbestos cement pipes or drinking water from a source high in naturally occurring
asbestos was suspected of causing increased risk of gastrointestinal cancer. However, recent comprehensive reviews
of the epidemiology does not show any increased risk of
gastrointestinal cancer or any other asbestos-related disease from drinking water contaminated with fibrous particles (Kanarek, 1989; Hillerdal, 1999; Browne et al., 2005;
Gamble and Gibbs, 2008).
The preponderance of evidence from experimental animal studies with rats and hamsters living a lifetime with
asbestos in their food have shown little or no increased risk
of gastrointestinal cancer or any other disease of the gastrointestinal tract (Moore, 1978; McConnell et al.,
1983a,b). Nor have experimental animals exposed to asbestos by inhalation developed such diseases. The lack of evidence for the ingestion or inhalation of asbestos causing
gastrointestinal cancer in experimental animals leads to
questions about causality in those ecological studies of
asbestos ingestion where a small effect is shown (Doll,
1989). Gastrointestinal cancer involves cancer at a number
of sites the most common of which is stomach cancer.
Markedly different incidence rates of this disease exist
between different regions of the world and between different races living in the same city (Higginson et al., 1992).
Although on the decline it remains the most frequently
occurring human cancer in the world and we have no idea
of the cause(s) so it is difficult to understand the reasons for
fluctuation in the incidence rates. Similarly the World
Health Organization and other national and international
health organization emphasized the extremely low-risk of
health effects from asbestos in water (WHO, 1986, 1989,
1999). On the basis of current knowledge no epidemiological study support a claim of adverse health effects from
disposing of waste rock in the lake.
When President Ronald Reagan signed the Asbestos
Hazard Emergency Response Act (AHERA) into law in
1986, interest again began to focus on monitoring asbestos
levels in indoor air post-asbestos abatement. A direct preparation technique was adopted for the preparation of the
air samples and analytical transmission electron microscopy (ATEM) would be used for the fiber analysis (ISO,
1995). ATEM is useful when a significant percentage of
the airborne fibers present are not asbestos and it is important to know the concentration of very short and/or thin
fibers. AHERA air sampling strategy requires the direct
counting of fibers greater than or equal to 0.5 lm in length
(one tenth of the OSHA exposure index fiber) and a slightly
higher value for the length to diameter ratio of 5:1 or
greater. The air was sampled at approximately 16 l of air
5
per minute but only for 4–6 h rather than the 72 h used
in Silver Bay.
In 1986, the United States Environmental Protection
Agency published the Airborne Asbestos Health Assessment
Update. The risk assessment is derived from the increased
incidence of lung cancer and mesothelioma among cohorts
occupationally exposed to asbestos. The assessment uses
the occupational exposure index noted above. It assumes
linear (no threshold) dose–response curve. The risk calculated using the Airborne Asbestos Health Assessment
Update would be accurate if one were to assume the fiber
exposures in Silver Bay were all equivalent to the average
asbestos potency derived from the results of epidemiology
studies of asbestos-exposed cohort of workers. This assessment is also the basis for the overall risk coefficient for
asbestos-related cancer risk listed in the Integrated Risk
Information System (IRIS) (see United States Environmental Protection Agency (IRIS) http://www.epa.gov/
iris/subst/0371.htm) which became available in 1988. Neither the air sampling protocols for non-occupational exposure nor the asbestos-related cancer models were available
at the time of the Reserve Mining Case.
4. United states consumer product safety commission and the
occupational safety and health administration address
cleavage fragments
The most interesting development since 1975 has been
the results of a geological survey showing that the Northshore fibers associated with taconite ore are an assortment
of cleavage fragments and alteration products and not
asbestos (Ross et al., 2008a,b). Associated with this is the
determination that such cleavage fragments are less potent
in producing human cancer as well as cancer in experimental animals (Davis et al., 1991; Nolan et al., 1991; Federal
Register, 1992; Ilgren, 2004; Gamble and Gibbs, 2008).
The definition used for the regulation of asbestos has
been an issue at least since 1971 (Federal Register, 1971).
The morphological counting criteria that accompanies the
definition of asbestos used in the OSHA regulations has
been incorrectly as including certain types of rock fragments occurring in a fibrous form commonly called cleavage fragments and alteration products as if these fiber
were asbestos (Langer et al., 1991). The fibers had to be
one of the six regulated asbestos minerals, visible by
phase-contrast microscopy, greater than or equal to 5 lm
in length and have a length to width (or aspect ratio) of
3:1 or greater. Several different types of mines (including
St. Lawrence tremolitic talcs in New York State, vermiculite from Enoree, South Carolina and taconite) had fibers
meeting morphological counting criteria, which were not
asbestos. Some in public health researchers became concerned that exposures to potentially dangerous fibers
would be ignored due to the arcane mineralogical criteria
defining asbestos, which were not relevant to any health
hazard evaluation (Health Effects of Tremolite, 1990;
Nolan et al., 1991; Federal Register, 1992). This position
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was in part rationalized by the results of experimental animal studies by Stanton et al. (1981) showing the importance of morphology.
The new scientific evidence since 1971 has weakened the
rationale for regulating non-asbestos amphibole fibers
under the asbestos standard. The new evidence has been
obtained from high-resolution transmission electron
microscopy examination revealing unique structural properties of asbestos, experimental animal studies showing
much lower risk for non-asbestos amphibole fibers compared with amphibole asbestos and epidemiological studies
showing little or no increase in asbestos-related diseases
among workers occupationally exposed to non-asbestos
amphibole fibers (Langer et al., 1991; Nolan et al., 1991;
Veblen and Wylie, 1993; Ross et al., 1993; Ross and Nolan,
2003; Ilgren, 2004; Gamble and Gibbs, 2008).
At various times OSHA had administrative orders,
which limited the impact of asbestos regulation on nonasbestos fibers, although these could be reversed at the discretion of the agency. This loose end of the asbestos regulations led to a claim in 1986 that tremolite asbestos was
present in children’s play sand. This issue was addressed
by the Consumer Product Safety Commission (CPSC)
(Germine, 1986, 1987; Langer and Nolan, 1987). The
CPSC found that tremolite in the play sand was not asbestos but rather cleavage fragments. Of the 2–4% tremolite
asbestos originally claimed in the New England Journal
of Medicine to be present approximately 0.01% of the
tremolite was in the form of fibers with size distributions
similar to asbestos. Once these two types of fibers are
recognized to be different and the health effects of the
non-asbestos fibers are separated those of asbestos the
non-asbestos fibers are clearly less active than asbestos
therefore should not be regulated using the asbestos permissible exposure limit (Langer et al., 1991; Nolan et al.,
1991). After careful review and public hearing both CPSC
and OSHA found insufficient cause to regulate non-asbestos fibers (Federal Register, 1992). In the public hearings,
Terence Scanlon, the Chairman of the CPSC at the time,
likened calling cleavage fragments asbestos to hollering fire
in a crowded theater.
The disharmony between these two federal regulatory
agencies concerning the types of fiber that should be regulated as asbestos put further pressure on OSHA to re-evaluate this long-standing matter and render some type of
final decision. Public hearings were conducted by OSHA
to gather information concerning the matter as well as written comments and documents submitted to the rulemaking
docket. After a careful review, of over two years, the
agency’s final rule appeared in the Federal Register on June
8th, 1992 (Federal Register, 1992). The non-asbestos
amphibole minerals were not to be regulated as asbestos.
National Institute of Safety and Health (NIOSH) had recommended that ‘‘. . . for regulatory purposes that cleavage
fragments of the appropriate aspect ratio and length from
the non-asbestiform mineral should be considered as hazardous as fibers from the asbestiform minerals” OSHA dis-
agreed with NIOSH’s recommendation and stated ‘‘. . .
OSHA does not believe the current record provides an evidentiary basis to determine ‘‘the appropriate aspect ratio
and length” for determining pathogenicity.” OSHA concluded ‘‘. . . the discussion indicates that populations of
fiber and populations of cleavage fragments can be distinguished from one another when viewed as a whole. For
example, one can look at the distribution of aspect ratios
or even widths for a population of particles as being asbestiform or non-asbestiform. However when one looks at
individual particles (e.g., particles from air sampling filters)
sometimes these mineralogical distinctions are not clear.”
Later in our report we will show Northshore fibers have
population characteristics which are not consistent with
asbestos, but are consistent with a population of nonasbestos fibers.
OSHA also concluded ‘‘. . . for most mineral deposits,
asbestos and non-asbestiform habits are distinguishable.”
‘‘OSHA has determined that non-asbestiform ATA and
asbestos anthophyllite, tremolite and actinolite should be
defined separately for regulatory purposes to conform to
common mineralogic usage.” The rule making focused on
only three of the five amphibole asbestos minerals regulated under the asbestos standard. The reason for this is
that the commercially less important anthophyllite, tremolite and actinolite asbestos but not have specific have specific commercial asbestos names. Therefore grunerite
asbestos and riebeckite asbestos known commercially,
respectively, as amosite and crocidolite were not included
in the rulemaking. Although logically the non-asbestos
fibers formed by any of the five amphiboles should not
be regulated as asbestos.
OSHA concluded that mineral fibers should be regulated based on using mineralogical criteria to define them
rejecting the similarity in morphology as an acceptable criteria for inclusion in the asbestos standard. This regulatory
action eliminated any justification for claiming a federal
definition for asbestos that differs from the mineralogical
definition described by Ross et al., 1984, 2008b. The OSHA
ruling eliminated any justification for claiming a federal
fiber definition.
OSHA concluded that, ‘‘. . .currently available evidence
is not sufficiently adequate for OSHA to conclude that
these mineral types pose a health risk similar to asbestos.” Although the non-asbestos amphibole fibers were
not shown to be non-carcinogenic the evidence available
was adequate to demonstrate their carcinogenic potency
was clearly less than that of asbestos. OSHA recognized
that a small percentage of populations of cleavage fragments and asbestos would be indistinguishable but
accepted that each is a unique mineral with different
potential for causing a health hazard. One critical point
is that it is difficult to find environments where a highconcentration of non-asbestos amphibole fibers is present
in the air. This is consistent with the information available for fibers released at the Northshore facility in Silver
Bay, Minnesota.
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Based on the information that became available after
the 1975 decision in the Reserve Mining case we decided
to take the following steps to fill in the data gaps which
existed at that time:
1. We undertook a geological survey of the Peter Mitchell
Pit, near Babbitt, Minnesota focused on determining if
asbestos and/or other fibrous minerals are present. A
summary of the geological survey will be presented
here the details are in Ross et al. (2008a).
2. Conduct environmental air sampling in Silver Bay, Minnesota to determine the concentration and type of airborne fibers that were greater than or equal to 5 lm in
length.
3. Compare the airborne concentration of fiber in Silver
Bay to the background ambient airborne levels of
asbestos fiber worldwide as reported by the World
Health Organization.
4. Determine the size distribution of airborne fibers in Silver Bay and compare the size distribution to respirable
grunerite asbestos (amosite). Using the criteria OSHA
described in 1992 that on a population basis cleavage
fragments have a size distribution different from respirable airborne asbestos (Federal Register, 1992).
5. The risk assessment model used here to evaluate the risk
of asbestos-related disease in Silver Bay, Minnesota is a
simple linear relationship between dose and response.
Several alternate risk coefficients for this relationship
were considered. First a coefficient derived from EPA’s
Integrated Risk Information System (IRIS). Then we
used the coefficients suggested by Hodgson and Darnton (2000) to evaluate each type of asbestos-related cancer separately and specifically for the type of asbestos
most similar in elemental composition to the airborne
fibers at Silver Bay, Minnesota and for the least active
of the asbestos fiber types—chrysotile asbestos. The
assumptions are described in detail below.
6. Critically review the epidemiological and non-human
primate studies designed to determine if grunerite
asbestos (amosite) causes mesothelioma at exposures
below the historical occupational exposures.
5. Geological survey of the peter mitchell pit
As noted above, asbestos is a mineralogical and economic geology term which is used to describe a highly
fibrous group of commercial minerals (Ross et al., 1984,
2008a). These minerals form in rather high-concentrations
as seams in dilated rock. These seams can vary from
approximately 1 mm to several centimeters in width.
Before measuring the airborne fiber concentrations in Silver Bay, 49 bulk samples were collected from the Peter
Mitchell Pit near Babbit, Minnesota to survey for asbestos
in the ore. A priority was given to examining areas in the
pit where there appeared to be geological faults and shear
zones looking for slip fibers along limbs of tight folds.
7
Asbestos can form in these areas and in addition the
amphibole minerals within these types of rock are particularly susceptible to a geological process called weathering
which occurs via low-temperature alterations due to the
infusion of rain water coupled with oxidation and rock
shearing. Two types of alteration of amphiboles minerals
were noted:
1. The Type I samples contained the amphibole ferroactinolite that has partially altered to very fibrous crystallites,
red-brown in color indicative of a hydrous iron oxide
mineral. The fibrous crystallites usually form as a mass
of fiber bundles. In the incompletely altered material,
amphibole grain areas can be seen within the fiber bundles where the ferroactinolite is green in color, nonfibrous and pristine. It is suggested that rain water moving through the shear zones altered and oxidized the original amphibole to a red-brown acicular product. Where
the alteration is incomplete, some of the pristine amphibole remains. It also appears that some iron was
removed from the amphibole grains during this weathering process to recrystallize as iron oxide, probably in
the form of goethite [FeO (OH)] which can form on
grunerite asbestos (amosite) too, and appears as brown
masses associated with the weathered fibrous material.
Examination by analytical transmission electron
microscopy of two samples representative of Type I
reveal fibers similar to those of the ferroactinolite used
by the EPA in experimental animal studies (Coffin
et al., 1983). After injection of the ferroactinolite into
the experimental animals the fiber number increased
with time by separating along the altered zones producing mesothelioma in the rat.
2. The Type 2 samples contained ferroactinolite amphibole that is much more altered than Type I. The
amphibole crystals have been degraded to a ropy to
platy mass with only a small amount of the original
material left. The platy mass gives X-ray diffraction
lines that suggest the amphibole is altered to ferrosepiolite or hydrobiotite. Examination of two Type 2
samples by analytical transmission electron microscopy reveal highly fibrous minerals with an elemental
composition similar to sepiolite and unlike any regulated asbestos mineral.
The conclusion of the fiber survey is that only a tiny
fraction of less than 1% of the total rock mass in the
Peter Mitchell Pit is fibrous. The fibrous ferroactinolite
is a low-temperature alteration product of non-fibrous
amphiboles; it does not occur in the manner of common
commercial asbestos, which crystallizes as a primary mineral from hydrothermal solution into open veins within
deformed rock. There was no evidence of a geological
process of alteration for cummingtonite-grunerite forming fibers similar to the ferroactinolite. No primary
asbestos minerals were found in the Peter Mitchell Pit
(Ross et al., 2008a).
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R. Wilson et al. / Regulatory Toxicology and Pharmacology xxx (2008) xxx–xxx
6. Risk assessment for asbestos
6.1. General comments
If the distribution of the asbestos fiber types and morphology were the same in the Northshore environmental samples
as in the various occupational asbestos exposures from
which the risk coefficients were derived, the risk assessment
would be relatively straightforward. But that is not the case
(Gamble, 2008; Nolan et al., 2008). The risk coefficients are
indexed to the fibers greater than or equal to 5 lm in length
having length to width ratios of 3:1 or greater and shortfibers are not counted. All of the coefficients in the risk assessment models for asbestos-related cancer are derived from
occupational exposure to airborne asbestos where only fibers
equal to or greater than 5 lm in length are counted. EPA
takes an average value for the various types of asbestos fiber
and does not in the risk assessment model address the question of whether some asbestos fibers types have different
potency nor do they address the potency of non-asbestos
amphibole fibers. Hodgson and Darnton (2000) reanalyze
the data (including several new epidemiology reports particularly on amphibole asbestos-exposed cohorts) and conclude there are large differences between the various
commercial asbestos fiber types that should be considered.
However they do not address cleavage fragments or other
types of non-asbestos fibers. Our analyses of Northshore
fibers indicate that on a population basis their size distribution is consistent with non-asbestos amphibole fibers rather
than asbestos. Because these are still issues being discussed
(and may therefore be considered controversial) we estimate
the risk by using several different risk coefficients to illustrate
how fiber type impacts the outcome. We note that under all
reasonable possibilities the risk of asbestos-related cancer
from environmental exposure is small. There should be no
controversy about this statement.
6.2. EPA-IRIS aggregate risk coefficient
We calculate an aggregate lifetime risk using the EPAIRIS system, noted earlier, that provides a summed risk
for two asbestos-related cancers. The risk is an average
of the risk to a standard US population. This uses an
‘‘absolute risk” model for mesothelioma and a ‘‘relative
risk” model for lung cancer. It is conservative (or one
might say pessimistic) in that the model assumes a linear
no threshold (LNT) increase in cancer risk.
According to the LNT model any exposure, no matter
how small, increases the aggregate cancer risk and the
model assumes continuous exposure over a 70-year lifetime. As the model is linear very small increases in exposure
correspond to very small increases in the cancer risk. The
model predicts an average risk for exposure based on
asbestos-related cancer among workers occupationally
exposed to the three principal commercial asbestos fiber
types—chrysotile, riebeckite asbestos (crocidolite) and
grunerite asbestos (amosite) and indexes the relative risks
to fibers greater than or equal to 5 lm. Asbestos fibers less
than 5 lm are not included in the exposure index. The
EPA-IRIS model uses the following equation:
Screening value ¼ Target cancer risk=Inhalation unit risk
% Screening value (SV) is the exposure to fibers equal to or
greater than 5 lm in length given as f/mL, at which the
Risk equals the Target cancer risk.
% Target cancer risk (TR) is the lifetime cancer risk for
example, 1 asbestos-related cancer death in 10,000 lifetimes is reported as a frequency of how often it occurs—
0.0001 or 10"4.
% Inhalation unit risk (IUR) is the upper bound excess lifetime cancer risk estimated to result from continuous lifetime exposure to asbestos given in the EPA-IRIS as
0.23 mL/f.
SV ¼ TR=IUR ¼ 0:0001=0:23 ¼ 0:0004 f=mL
Using the EPA-IRIS risk coefficient we predict one asbestos-related cancer death over the lifetimes of 10,000 people
exposed continuously for 70 years to an average daily
asbestos exposure of 0.0004 f/mL. It is important to realize
that it could never be proven directly that a risk of this
small magnitude exists.
7. Exposures to fibrous minerals in Silver Bay, MN
The EPA-IRIS aggregate risk coefficient assumes that the
exposure is estimated from the concentration of fibers
greater than 5 lm with an aspect ratio of 3:1 or greater. This
exposure had not previously been determined for the residents of Silver Bay as the air sampling mandated by the court
case counted fibers with lengths less than 5 lm and is therefore not useful for risk assessment. To determine the concentration and exposure we selected the air sampling station
closest to the residential area because it would be most representative of the level of exposure the residents would experience. Twelve air samples were collected between October 24
and December 9, 1998. Three of the air samples were collected in duplicate therefore nine values of the average fiber
concentration greater than or equal to 5 lm over a 24-hour
period were determined. Each of the air samples was prepared by the direct transfer technique for examination by
analytical transmission electron microscopy using the protocol described in ISO, 1995. The grid openings were scanned
directly on the screen at 20,000! magnification. Any object,
which had a length three times greater than its width was considered a fiber. Energy dispersive X-ray spectra were
obtained for each fiber and a digital image was recorded.
The fibers were sized from the printout of the digital image.
Only two fiber types having an elemental composition similar to any of the asbestos minerals were found and used for
the exposure estimates. These were cummingtonite-grunerite
(79%, N = 15) and tremolite-actinolite (21% N = 4). The
results of this analysis are shown in Tables 1 and 2.
The concentration on average was 0.00014 f/mL. This is
about 35% of the level IRIS predicts will cause one cancer
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0.00011
9
death in 10,000 lifetimes (Table 2). We assume that the residents of Silver Bay will be exposed to this concentration for
their lifetimes. According to the linear no threshold dose–
response relationship the lower exposure determined here
will be associated with 1 excess asbestos-related cancer
death in about 28,500 lifetimes (or 35 asbestos-related
cancer deaths in 1,000,000 lifetimes). Considering the population of Silver Bay is about 2500 the pessimistic conservative model predicts that at worst one excess cancer death
related to fiber exposure in more than 10 lifetimes of the
entire Silver Bay population. Since about 22% of the US
population die of cancer by age 70 about 6240 cancer deaths
from all types of cancer would be expected in this time period, including about 10 background mesothelioma cases not
related to asbestos exposure (Price and Ware, 2004) and 627
lung cancer cases. The lung cancer deaths assume 2.2% lung
cancer mortality among residents of Silver Bay.
The IRIS asbestos-related cancer risks are based on
exposure to asbestos fibers. The air sampling protocol used
in Silver Bay determined the airborne fiber concentration,
which is predominantly non-asbestos fibers, is at the lowend of background measurements for asbestos fibers in
other communities. If present in Silver Bay, asbestos fibers
would be at an even lower concentration. On the basis of
the geological survey of the Peter Mitchell Pit we doubt
whether any significant fraction of airborne fiber is asbestos. The fibrous ferroactinolite associated with the alteration products found in the geological survey occur at
small concentration in the pit and only about 20% of the
Northshore fibers in the air samples have elemental compositions consistent with ferroactinolite and only a sub-population of these have morphology consistent with the
alteration products. The concentration of airborne fibers
in Silver Bay is at what World Health Organization reports
as the low-end of background for airborne asbestos (WHO,
1986, see Fig. 1).
To further examine the type of airborne fiber in Silver
Bay we looked at the size distribution of 387 fibers of all
lengths found in the ambient air in and around Silver
Bay and compared the size distribution with 288 fibers of
respirable grunerite asbestos (amosite) lofted for experimental animal studies (Hesterberg et al., 1999; McConnell
et al., 1999). We choose grunerite asbestos (amosite) for
comparison because approximately 80% of the airborne
fibers at Silver Bay had elemental compositions consistent
with cummingtonite-grunerite (Tables 1 and 2). The Northshore fibers increase in diameter, to a significantly greater
extent than asbestos, as the fiber length increases. We conclude from this observation that the airborne fibers in Silver Bay are predominantly non-asbestos fibers (Table 3).
8. Comparison with EPA led task force working group doing
risk assessment for asbestos exposure world trade center
post-9/11
Sub-total
T7
T7
T7
F7
T7
11/17/98
11/21/98
12/03/98
12/09/98
Sub-total
Mean = < 0.00014 (n = 9)
0.00014
28,578
0
1
1
2
4
1
3
120
51,970
1.452
0.000035
<0.000071
0.00007
<0.000069
0.00021
0.00014
0.00036
0.00007
0.000069
0.00021
28,097
14,016
13,558
14,439
14,139
0
0
0
0
0
0
0
0
0
1
3
5
0
1
0
1
0
1
0
2
4
5
1
1
3
3
5
0
1
0
1
0
1
0
3
120
60
60
60
60
25,548
25,488
24,656
26,258
25,712
1.456
0.726
0.726
0.726
0.726
<0.000074
0.00019
27,229
2
0
3
0
2
5
0
121
49,116
1.464
0.00024
0.00029
<0.000073
0.00028
<0.000073
<0.000074
0.00067
0.0008
<0.000073
0.00099
0.00022
0.00015
25,110
13,823
13,607
14,209
13,647
13,582
5
1
0
2
1
1
2
0
0
1
0
0
6
6
0
8
2
1
4
4
0
3
0
0
17
11
0
4
0
2
11
7
0
10
3
2
6
4
0
4
0
0
1.673
0.738
0.726
0.726
0.726
0.738
113
61
60
60
60
61
48,509
24,727
24,744
25,839
24,818
24,298
T7-1
T7
T7
F7
T7
Sub-total
10/28/98
10/30/98
11/05/98
11/10/98
0.00026
0.00022
0.00051
0.00082
6
11
2
2
1
5
1
1
2
3
11,729
13,391
<5 lm
3
8
3
3
Electron microscopy in the area of the Northshore pellet plant
10/24/98
F7-1
24,157
53
0.641
F7(T)-1
24,352
60
0.726
<5 lm
>5 lm
<5 lm
>5 lm
>5 lm
Grid area
examined
(in mm)2
N" of fields
examined
Volume of
air (in liters)
ID N"
Date
Table 1
Results of the analysis of site N" seven air samples by analytical transmission
Total
Cummingtonitegrunerite length
N" of fields
detected length
Tremolite
actinolite length
Volume of air
scanned (mL)
Total airborne
fiber concentration
F/mL all lengths
Total airborne fiber
concentration F/mL
with length >5 lm
R. Wilson et al. / Regulatory Toxicology and Pharmacology xxx (2008) xxx–xxx
We now compare our approach to that of the EPA for
the World Trade Center post-9/11. The initial air sampling
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R. Wilson et al. / Regulatory Toxicology and Pharmacology xxx (2008) xxx–xxx
Table 2
Summary of the results of the air sampling at station N" seven for risk assessment
Air sample
1
2
3
4
5
6
7
8
9
Weather
No precipitation
No precipitation
0.0600 Precipitation
No precipitation
1.9600 Precipitation
No precipitation
No precipitation
No precipitation
No precipitation
N" of fibers detected
length >5 lm
Fiber type
Cummingtonite/
grunerite
Tremolite/
actinolite
6
4
0
4
0
1
0
1
3
4
4
0
3
0
1
0
1
2
2
0
0
1
0
0
0
0
1
Volume of air
scanned (in mL)
Total airborne fiber concentration
f/mL with length >5 lm
25,110
0.00024
13,823
0.00029
13,607
<0.000073
14,209
0.00028
27,229
<0.000074
28,097
0.000035
14,016
<0.000071
13,558
0.00007
28,578
0.00011
Mean<0.00014 ± 0.001
Table 3
Comparison of the length distribution of grunerite (Amosite) asbestos and the 387 airborne fibers of cummingtonite-grunerite and tremolite-actinolite
collected at the perimeter of the Northshore pellet plant
Samples
Fibers sized
a
N" of % in each length range
<5 lm
>5 to <10 lm
P10 to <20 lm
620 lm
Grunerite(Amosite) asbestos
Average diameter
Average aspect ratio
288
288
288
47.2
0.27 ± 0.17 lm
15 + 12
23.6
0.39 ± 0.30 lm
27 ± 22
15.6
0.37 ± 0.29 lm
57 ± 40
13.5
0.70 ± 0.75 lm
120 + 116
Northshore fibers
Average diameter
Average aspect ratio
387
65.9
0.60 ± 0.27 lm
6.2 ± 3.7
23.5
1.13 ± 0.56 lm
9.1 ± 10.9
9.6
2.12 ± 1.02 lm
8.4 ± 7.1
1.0
3.65 ± 2.63 lm
10 ± 5
The aspect ratios of the Northshore fibers are independent of length while the aspect ratio of a population of grunerite (amosite) asbestos fibers increases
with length. This difference is the morphological basis for differentiating asbestos and cleavage fragments.
*
Determine from photographs and digital images obtained by transmission electron microscopy.
a
The grunerite (amosite) asbestos used for comparison is a respirable sample lofted for the studies by Hesterberg et al., 1999; McConnell et al., 1999.
undertaken by a multi-agency Task Force focused on outdoor air (World Trade Center, 2003). Measurements of airborne asbestos concentrations in Lower Manhattan post-9/
11 were evaluated against a 70 structures per millimetersquared standard, which corresponds to 0.021 f/mL counting all fibers greater than 0.5 lm with aspect ratios of 5:1
(Office of Inspector General, 2003). Once asbestos exposures were below this level EPA recommended that residents be allowed to return to their homes in Lower
Manhattan. The outdoor concentrations of asbestos fibers
considered acceptable in Lower Manhattan were 58-fold
higher than the mean for the predominantly non-asbestos
concentrations of fibrous particle of all lengths associated
with taconite that were measured in Silver Bay (Table 1).
The EPA value is not based on lifetime asbestos-related
cancer risk but rather it was derived from the background
contamination on the membrane filters used to collect the
air samples. Values up to 0.021 f/mL could be simply contamination on the filter and do not represent the airborne
asbestos level. Concern about the high-background contamination on the membrane filters date to the period of
the AHERA Act in 1986, our controls indicate contamination is much lower and generally chrysotile asbestos (Nolan
and Langer, 2001). If the concentration of asbestos in the
ambient air in Lower Manhattan was below this level it
could be considered acceptable based on the fact that the
debris removal would only last a year and the consequent
exposure would be acceptable for that period of time.
Exposure to a concentration of 0.021 f/mL for one year
gives the same cumulative exposure as exposure to
0.0003 f/mL for a 70-year lifetime. Not all structures are
fibers and the fiber counting criteria use a length of
0.5 lm or greater therefore the exposure is more protective
than 1 asbestos-related cancer death in 10,000 lifetimes.
After 9/11 the EPA decided to assist residents in cleaning of their apartments in Lower Manhattan. The agency
used the IRIS model to set a health-based benchmark for
when the apartments would be cleaned to an acceptable
standard (World Trade Center, 2003). The Task Force
decided that 1 excess asbestos-related cancer in 10,000 lifetimes would be the Target Cancer Risk (TR) corresponding
to a Screening value (SV) of 0.00043 f/mL for asbestos
fibers with a length greater than or equal to 5 microns.
The Task Force further concluded that the arithmetic mean
background concentration was between 0.00003 and
0.0060 f/mL and decided a residence would be clean and
acceptable for re-occupation if some form of aggressive
air sampling demonstrated the airborne concentration of
asbestos to be less than 0.0009 f/mL (accepting a concentration of airborne asbestos about 6-fold higher in Lower
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Manhattan apartments than ambient airborne fibers in Silver Bay). This Lower Manhattan exposure corresponds to
1 excess asbestos-related cancer in 10,000 lifetime for 35
years of continuous exposure rather than the 70 years corresponding to a Screening value (SV) 0.00043 f/mL having
a length greater than is equal to 5 lm.
Therefore the upper limit of predominantly non-asbestos
fiber concentrations in Silver Bay is six times lower than the
post-asbestos clean up level for asbestos fibers EPA used
for residents near the World Trade Center.
9. Hodgson and Darnton (2000) asbestos fiber type specific
risk coefficients for each asbestos-related disease
Approximately 80% of the airborne Northshore fibers in
Silver Bay have elemental compositions in the cummingtonite-grunerite series. The remaining fibers are in the tremolite-actinolite series. For the Hodgson and Darnton
(2000) risk assessment we will make two alternate assumptions that we did not need to make for the IRIS model. Initially we will assume that all the airborne fibers in Silver
Bay are as potent in producing asbestos-related cancer as
the asbestos fiber type with the same elemental composition. Then we will calculate the asbestos-related cancer risk
assuming the Northshore (non-asbestos) fibers are only as
potent as the least dangerous form of commercial asbestos
which is chrysotile asbestos. This last assumption seems
consistent with the conclusion by CPSC and OSHA that
the non-asbestos fibers are less active than asbestos.
First, we will assume the Northshore fibersare all grunerite asbestos (amosite) as approximate 80% of Northshore
have elemental compositions in the cummingtonite-grunerite series and there is no risk assessment model yet available for tremolite-actinolite asbestos. For this we use
Hodgson and Darnton’s coefficient for grunerite asbestos
(amosite). We would argue that this assumption seriously
over estimates the risk for asbestos-related disease among
the long-term residents of Silver Bay. It was the default
assumption during the period of the Reserve Mining controversy and represents the upper limit of an asbestosrelated cancer risk. Our second assumption is that the
Northshore fibers are no more active than the least active
asbestos fiber types. We make this assumption consistent
with OSHA’s conclusion that non-asbestos fibers are less
active than asbestos (Federal Register, 1992). At this time
we will not make any statement as to how much less the
Northshore fibers are than the least potent asbestos fiber
type. Accordingly we calculate two alternate values for
the mesothelioma risk the total expected mesothelioma
mortality (RM) expressed in fiber/mL ! years. We choose
the total value for amosite cohorts to predict the mesothelioma incidence if the Northshore fiber were all grunerite
asbestos(amosite) (RM = 0.1 fiber/mL ! years) (alternate
1) and the total chrysotile excluding South Carolina textile
workers assuming Northshore fibers to be equivalent to the
least potent asbestos fiber type (RM = 0.001 fiber/
mL ! years) (alternate 2).
11
The number of asbestos-related mesothelioma cases
(OM) depends on the type of asbestos to which one is
exposed, the cumulative exposure and the age at which
exposure first occurs (Hodgson and Darnton, 2000) and
can be calculated by:
OM ¼
RM ! ECA ! Tpop
100
Where:
% RM—Risk of mesothelioma as a percentage of the total
expected mortality per (f/mL) year. The RM used, 0.1, is
obtained from Hodgson and Darnton, 2000 (entry ‘‘Total
amosite cohort” in their Table 1) (adjusted to 30 years of
age at first exposure) the value of RM is specific for grunerite asbestos (amosite). This is derived from occupational
exposure, assumed to be 8 h/day for 250 days per year.
% ECA—The environmental concentration of grunerite
asbestos (amosite) is 0.00014 f/mL and needs to be converted to an occupational concentration from which the
risk coefficients are derived. We assume an 8 h/day for
250 days per year. This is done by taking the environmental concentration (0.00014 f/mL) and multiplying
by 4.38 to the equivalent occupational concentration
of 0.00061 f/mL. We assume this exposure goes on for
40 years so we multiply 0.00061 f/mL by 40 to obtain
the cumulative concentration of 0.0245 f/mL ! years.
% Tpop—Adjusted total exposed population for Silver Bay.
The total population is 2500 residents.
This equation is basically the same equation as in Hodgson and Darnton, 2000, page 566 but there X is used in
place of Eca and Eadj is the total expected deaths from all
causes which we here set equal to the total population Tpop
on the plausible assumption that everyone will die sometime. It is similar but with different notation to the equation used with the EPA-IRIS risk coefficients.
By solving for OM we find that 0.061 mesothelioma
cases among the 2500 residents of Silver Bay (or 24 mesothelioma cases per 1,000,000) might be caused by the total
fiber exposures. This assumes that the Northshore fibers
are all as potent in producing cancer as grunerite asbestos
(amosite) from which we derived the lifetime risk of 24
mesothelioma cases in 1,000,000 lifetimes (alternate 1).
Assuming, however, that the Northshore fibers are only
as active in producing mesothelioma as the least potent
asbestos fiber type which is chrysotile asbestos this estimate
is reduced to 0.24 mesothelioma cases in 1,000,000 (alternate 2). This can be compared to the background rate of
mesothelioma not related to asbestos exposure which has
recently been estimated to be 350 cases in 1,000,000 lifetimes (Price and Ware, 2004).
Again we select two values this time for the lung cancer
risk (RL) the percentage expected lung cancer mortality
expressed as fiber/mL ! years. We chose the total value
for amphibole asbestos [exposure to riebeckite asbestos
(crocidolite) and/or grunerite asbestos(amosite)] cohorts
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R. Wilson et al. / Regulatory Toxicology and Pharmacology xxx (2008) xxx–xxx
[excluding Republic of South Africa (SA) miners] to predict
the percentage increase in lung cancer assuming the Northshore fiber were as potent as amphibole asbestos (RL = 5.
1 fiber/mL ! years) (alternate 3) and the best estimate of
the lung cancer risk for chrysotile asbestos assuming
Northshore fiber to be equivalent to the least potent asbestos fiber type (RL = 0.1 fiber/mL ! years) (alternate 4).
For a given cumulative asbestos exposure, the risk of
developing lung cancer will increase as a percentage of
the existing lung cancer risk in the population. We will
assume that on average 8% of cigarette smokers develop
lung cancer, 90% of the lung cancers are found in smokers,
and 25% of the residents of Silver Bay smoke. Lung cancer
mortality in Silver Bay would be 2.2%. The risk of lung
cancer increases linearly with cumulative asbestos exposure
following the relationship:
RL ! ECA ! ExpL
100
2:2025% ¼ 2:20000% þ 0:0025%
ObsL ¼ ExpL þ
We calculate the increase in the observed number of asbestos-related lung cancers (ObsL) assuming exposure to
Northshore fibers is as potent as asbestos.
% RL—Risk of lung cancer expressed as a percentage of lung
cancer deaths per f/mL ! years of asbestos exposure. The
RL used is 5.1 obtained from Hodgson and Darnton, 2000
(entry ‘‘ex. SA amosite” in their Table 2) and although an
average is specific for amphibole asbestos.
% ECA—The cumulative chrysotile asbestos environmental
concentration (assumed to be continuous) 0.00014 f/
mL ! years is converted to the equivalent occupational
concentration of 0.00061 f/mL ! years. Assuming 40
years of exposure the ECA is 0.0245 f/mL ! years.
% ExpL—Expected background of lung cancer deaths,
55.5, among the 2500 residents of Silver Bay. This background rate is determined by solving equations that
reflect the relationship between the percentage of smokers who get lung cancer and the percentage of lung cancers that occur in smokers. Specifically, 0.9 ! (N" of
lung cancers) = 0.08 ! (Silver Bay Population)
= 0.08 ! 0.25 ! 2500/0.9 = 55.5.
Using these values ObsL = 55.5 lung cancers expected
plus 0.069 of a case increase from assuming Northshore
fibers are as potent as amphibole asbestos. The increase is
equivalent to 27 lung cancer cases per 1,000,000 lifetimes
(alternate 3). The lung cancer risk falls to 0.53 cases per
1,000,000 assuming the Northshore fibers are as potent as
the least active form of asbestos (chrysotile with a
RL = 0.1 fiber/mL ! years) (alternate 4).
10. Conclusions
The total risk for the asbestos-related cancers assuming
the Northshore fibers have a potency equivalent to that of
grunerite asbestos (amosite) and the least active asbestos
fiber type—chrysotile asbestos—is, respectively, 51 or
0.77 asbestos-related cancers per 1,000,000 lifetimes
according to Hodgson and Darnton (2000) or 1 asbestosrelated cancer in 28,500 lifetimes according to EPA’s IRIS
an average for all the asbestos fiber types which corresponds to 35 asbestos-related cancers in 1,000,000 lifetimes.
However we believe, as did the Consumer Product
Safety Commission and the Occupational Safety and
Health Administration, that the potency of non-asbestos
fibers to induce cancer is far less than the potency of asbestos to do so. Comparison of the size distribution of airborne Northshore fibers with respirable grunerite
asbestos (amosite) indicate the Silver Bay exposures are
predominantly non-asbestos fibers and not asbestos. Even
in South Africa were grunerite asbestos (amosite) was commercially mined for more then 70 years no environmental
mesotheliomas are known to occur and mesothelioma
among the large mining population has been and remains
a rare disease (Murray and Nelson, 2008).
There is only one epidemiology report in the world medical literature whose goal was to determine if asbestosrelated diseases were developing from neighborhood exposure to grunerite (amosite) asbestos.
We have established that the airborne fiber levels in Silver Bay are at the low-end of background for asbestos
worldwide (Fig. 1). Although the EPA-IRIS provides useful information and allows us to compare the fiber associated cancer risks in Silver Bay with EPA’s recent approach
to asbestos post-9/11 in Lower Manhattan, it does not
allow us to fully explore the issues about asbestos fiber type
that are critical to understanding cancer risk from Northshore fibers in Silver Bay (Nolan et al., 1999, 2005). As
asbestos risk assessments specific for fiber type are now
available (Hodgson and Darnton, 2000) we selected the
least potent form of asbestos for the upper limit of the
mesothelioma and lung cancer risk and found it to be less
than 0.77 cases per 1,000,000 lifetimes from exposure to
Northshore fibers in Silver Bay, Minnesota.
We further conclude that the steps to dispose of the
waste rock on land were sufficient to reduce the risk of cancer from fiber exposure among the general population
around the Silver Bay facility to a combined excess risk
of lung cancer and mesothelioma of approximately 1/
1000 of the background risk (Price and Ware, 2004). The
disposal method selected for the waste rock is highly likely
to prevent the build-up of fibers in the environment. Nothing has occurred to indicate that additional steps need to be
taken to further reduce the health hazards in Silver Bay
related to exposure to Northshore fibers.
We emphasize that the conclusions are based upon using a
model that has a linear dose–response relationship. However
many scientists believe that there is a threshold exposure
below which asbestos fibers do not cause cancer. If we were
to believe such a threshold model and the exposures in Silver
Bay below threshold, the predicted effects would approximately zero and our conclusions correspondingly enhanced.
Please cite this article in press as: Wilson, R. et al., Risk assessment due to environmental exposures to fibrous ..., Regul. Toxicol.
Pharmacol. (2008), doi:10.1016/j.yrtph.2007.11.005
ARTICLE IN PRESS
R. Wilson et al. / Regulatory Toxicology and Pharmacology xxx (2008) xxx–xxx
Acknowledgments
We acknowledge support from a Higher Education Advance Technology grant from New York State and the
International Environmental Research Foundation
(www.ierfinc.org) of New York, New York and assistance
from Cleveland-Cliffs, Cleveland, Ohio.
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